1. Field of the Invention
The present invention relates to an organic light emitting device having a cathode including a magnesium-calcium layer and a method for fabricating the same.
2. Description of the Background
In general, an organic light emitting device includes an anode, an organic emission layer disposed on the anode, and a cathode disposed on the organic emission layer. When a voltage is applied between the anode and the cathode, holes are injected into the organic emission layer from the anode and electrons are injected into the organic emission layer from the cathode. The holes and electrons that are injected into the organic emission layer are combined in the organic emission layer to create excitons, and the excitons are transitioned from an excited state to a ground state to emit light.
Here, the cathode should have a low work function in order to readily inject the electrons into the organic emission layer. A material that satisfies this condition is magnesium that has a work function of 3.46 eV. However, the magnesium cannot be used to form a stable organic light emitting device since it reacts with external oxygen or moisture.
In order to solve this problem, U.S. Pat. No. 4,885,211 uses a cathode having high ambient stability by forming the cathode with a magnesium-silver (Mg—Ag) alloy. As a result, it is possible to obtain an organic light emitting device with good lifespan characteristics. However, since Ag has a relatively high work function of about 4.28 eV, the magnesium-silver alloy cathode may have inferior electron injection characteristics compared to the magnesium cathode.
In U.S. Pat. No. 5,776,622 a lithium fluoride (LiF) layer is deposited onto an electron transport layer (Alq) and an Mg—Ag layer is deposited onto the LiF layer, thereby forming a bilayer cathode of LiF/Mg—Ag. The bilayer cathode of LiF/Mg—Ag may have improved electron injection characteristics when compared to a single layer cathode comprising Mg—Ag.
However, when a separate layer is formed to improve the electron injection characteristics of the cathode, the manufacturing cost may increase and throughput may be reduced.